Among the most widely used insecticides today are the pyrethrins, the active principle of pyrethrum flowers (Chrysanthemum cinerariaefolium), which have a high order of insecticidal activity and a low mammalian toxicity. The relatively high cost and the uncertain supply pf pyrethrins have encouraged attempts to prepare synthetic insecticides which retain the desirable properties of pyrethrins. It has long been known that synthetic products having a basic structural similarity to pyrethrins in that they are esters of certain substituted cyclopropanecarboxylic acids, in particular 2,2-dimethyl-3-(2-methylpropenyl)cyclopropanecarboxylic acid (which is also known as chrysanthemumic acid and will be so referred to herein), exhibit insecticidal activity of a significant order.
The wide market which pyrethrins and related synthetic insecticides enjoy today is due primarily to the discovery of certain additives which enhance the activity of these insecticides. These additives, commonly called synergists, are agents which may or may not themselves exhibit insecticidal activity, but which when combined with pyrethrins or related compounds produce new insecticides, having an effectiveness significantly greater than the sum of the effectiveness of the components when used separately. A great deal of time and effort has been devoted to the search for effective synergists. One of the most effective and most widely used of the pyrethrins synergists is the compound piperonyl butoxide, which is described in synergistic combination with pyrethrins in Wachs U.S. Pat. No. 2,550,737. Unfortunately, it has been found that many compounds which are excellent synergists for pyrethrins are not nearly as effective when used with allethrin or other synthetic cyclopropanecarboxylic acid esters.